Heat seekers Turtle embryos bask inside their shells to take advantage of temperature changes, according to researchers, suggesting that the ability to control their own destiny begins a lot earlier in life than previously thought.

It is well known that reptiles can control their body temperatures by moving to more thermally-favourable conditions, but it had been assumed that the behaviour only began after hatching.

The freshwater turtles normally lay their eggs in shallow nests on sloping riverbanks, where temperatures can vary depending on an egg's proximity to the surface and warmth from the Sun.

The researchers, whose work appears in the journal Proceedings of the National Academy of Sciences, used a combination of candling (transmitting light through the shell to determine the position of the embryo) and partial removal of the shell to examine the responses of embryos to shifting heat sources in both a controlled lab experiment and in semi-natural nests.

They found that embryos moved slightly upwards towards the Sun-warmed ground surface in nests built in flat ground, while turtles in eggs buried in a sloping bank, which received their heat from the side of the nest, shifted from one side of the egg to the other in search of heat.

"They are able to detect temperature gradients and move from stage 12 of development, or about 20 per cent of the incubation period," says Dr Weiguo Du, lead author on the study, adding that the embryos respond to the heat source within three days.

"The highest speed of movement calculated from our data was about 10 degrees per day," he adds.

Being able to thermoregulate may bring a number of advantages to reptile embyos: even small eggs like P. sinensis, which weigh less than 5 grams, can have a temperature difference of 0.8°C. This is enough to affect the rate of development and a hatchling's survival prospects if it fails to hatch at the same time as its nestmates, which can increase its risk of predation.

Controlling gender

Sex is also determined by temperature in many species of reptile including some turtles. While Du says it has not yet been determined whether P. sinensis experiences temperature-dependent sex determination (TSD), "Studies on thermal dependence of sex ratio in many TSD species have shown that sex ratios can change significantly ... within 1°C around the pivotal temperature at which the sex ratio of hatchlings is balanced."

"Our experiment showed that the mean temperature on the part of the shell closest to the heat source averaged almost 1°C warmer than the other side of the same egg," he says. "Therefore, we speculate that if the embryos of TSD species behave the same way as we found in P. sinensis, they may be able to control their own sex."

"We have long suspected that other things are happening in the egg/nest environment and this research offers great insights into understanding turtle embryology in relation to rising temperatures and changing environments," says turtle biologist Dr Mark Hamann from James Cook University, who is researching the impacts of climate change on Australian and South East Asian turtle species, but was not involved in this study.

"It will also allow us to predict development and mortality rates with greater resolution. Some species are more susceptible to higher temperatures than others, and some are likely to be more able to move around than others," he adds.

"If eggs are already pushing the upper thermal limits this research shows they might be able to move to a cooler spot in the egg and survive."